2015 September 1

1: CANDU Reactor

B. Rouben

McMaster University

Nuclear Reactor Physics

EP 4D03/6D03

2015 Sept-Dec

Outline

A quick look at the design of CANDU Reactors:

Reactor Assembly

Pressure Tubes

Fuel

On-Power Refuelling

Heat-Transport System

Moderator System

Reactivity Devices

2015 September 2

2015 September 3

Schematic of a CANDU Nuclear Power Plant

CANDU-6 Plant

Turbine Building

Reactor Containment Building

2015 September 4Reactor

CANDU-6 Reactor

1. Reactor face

2. Reactor coolant pump

3. Steam generator

4. Fuelling machine carriage

5. Moderator heat exchanger

6. Dousing water system

7. Dousing water tank 5

1

3

2

4

6

7

2015 September 5

CANDU Core Design

CANDU

• Natural-uranium fuel

• Heavy-water coolant

• Heavy-water moderator

• Separate coolant and moderator

• Pressure tubes

• Small, simple fuel bundle

• On-power refuelling

2015 September 6

CANDU-6 ReactorVault

2015 September 7

Pressure Tubes (Fuel Channels)

Calandria

Heavy-Water Moderator (between & around fuel channels)

Feeders

Reactor Assembly

The reactor assembly contains the reactor core and

the reactivity control devices. Major components of

the reactor assembly are:

Calandria Vessel

End-Shields

Shield Tank

Fuel Channels

Reactivity Control Devices

2015 September 8

Calandria Vessel

Low-pressure tank

Includes calandria tube and supports pressure tubes

Contains heavy water moderator

Contains reactivity control devices and shutdown systems

Embedded in light-water reactor vault (which provides radiation shielding)

2015 September 9

CANDU 6 Calandria with Pressure Tubes Installed

2015 September 10

2015 September 11

Calandria, Showing Fuel Channels

Pressure-Tube Core Design

Sub-divided reactor coolant system, no large

pressure vessel.

Cool moderator separated from hot coolant.

Zr-2.5%Nb pressure tubes constitute CANDU

‘pressure vessel’.

Individual pressure tubes are replaceable.

Modular component – allows scaling of reactor

size.

Zirconium alloy provides neutron economy.

Interstitial reactivity devices (between fuel

channels).2015 September 12

Main CANDU Reactor Systems

Reactor Assembly

Fuel and Fuel Channels

Heat Transport System

Moderator System

Reactivity Devices (Control & Safety

Systems)

2015 September 13

CANDU 6 Heat Transport System

2015 September 14

Steam Generator

STEAM GENERATOR

Tube Bundles

2015 September 15

Reactor Face End Fittings and Feeders

2015 September 16

Reactor Coolant Parameters

Outlet header pressure 10 MPa

Outlet header temperature 310ºC

Outlet header steam quality (max.) 4.0%

Inlet header temperature 266ºC

Secondary Side Conditions

Steam pressure 4.7 MPa

Steam quality <0.25% moisture

Feedwater temperature 187ºC

CANDU-6 Heat-Transport System Design

2015 September 17

CANDU Fuel

Natural uranium (~0.7% 235U).

High-density uranium oxide (UO2) fuel

pellets in Zircaloy-4 cladding.

Short (0.5 m) fuel elements arranged in

cylindrical fuel bundles.

2015 September 18

CANDU 37–Element Fuel Bundle

2015 September 19

Uranium Fuel Pellets

Zircaloy Fuel Sheath

CANDU-6 Reactor Assembly (Side View)

2015 September 20

Fuel Channel

12 Bundles per Channel

2015 September 21

1 Basic Cell of CANDU Reactor

D2O

Primary

Coolant

Gas Annulus

Fuel Elements

Pressure Tube

Calandria Tube Moderator

2015 September 22

On-Power Refuelling

Refuelling for long-term maintenance of reactivity: required because reactivity eventually decreases as fuel is irradiated (fission products accumulate and total fissile content decreases).

In CANDU 6, average refuelling rate ~ 2 channels per Full-Power Day (FPD), using the 8-bundle-shiftrefuelling scheme (8 new bundles pushed in channel, 8 irradiated bundles pushed out).

4-bundle-shift and 10-bundle-shift refuelling schemes have also been used in other CANDUs.

Selection of channels is the job of the station physicist.

2015 September 23

Fuelling machines at both ends of the reactor

remove spent fuel, insert new fuel

Moderator System

Low-temperature (< 80oC), low-pressure system.

Independent of reactor coolant system.

Normal heat removal is ~4-5% of full power.

Contains reactivity devices located outside of high-pressure heat transport system.

Potential heat sink if Emergency Core Cooling is unavailable during a Loss-of-Coolant Accident (LOCA).

2015 September 24

Moderator System

2015 September 25

2015 September 26

CANDU Reactivity Devices

All reactivity devices are located or

introduced into guide tubes permanently

positioned in the low-pressure moderator

environment.

These guide tubes are located interstitially

between rows of calandria tubes (see next

Figure).

2015 September 27

CANDU-6

Reactor

(700-MWe

Class)

Ion Chambers

Interstitial

Guide Tubes for

Reactivity

Devices (Zone

Controllers,

Adjusters, …)

2015 September 28

CANDU Reactivity Devices

For Regulation (Control):

14 liquid-zone-control compartments (H2O

filled)

21 adjuster rods

4 mechanical control absorbers

Moderator poison

For Emergency Shutdown:

2 Shutdown Systems: SDS-1 & SDS-2

2015 September 29

CANDU Special Shutdown Systems

Two independent,

fully capable

shutdown systems:

SDS-1 (cadmium

rods enter core

from top)

SDS-2 (injection of

gadolinium

neutron “poison”

from side.

2015 September 30

END